1. Field of the Invention
The present invention relates to a thin-type flat speaker.
2. Description of the Related Art
Recently information terminals are developed to be mobile style, and required to be thin, downsized, and light weighted. In order to satisfy the requirement, a flat speaker has been developed. For example, the flat speaker is developed in corresponding to the needs for a liquid crystal display television, and automobile to be thinner and light weighted.
An electro-conductive flat speaker may be thinner and downsized in the construction in comparison with a corn type speaker. An example of the construction of a conventional flat speaker is described with reference to FIG. 2. FIG. 2 is a cross sectional view of the conventional flat speaker.
The flat speaker depicted in FIG. 2 includes a vibrating membrane 2 with a voice coil 4 provided on one face or both faces of an insulating base film 3, a plurality of permanent magnets 5, and yoke 6 for installing the permanent magnets 5. The vibrating membrane 2 is held to a frame 8 by an edge 7 thereof. The voice coil 4 is formed on the insulating base film 4 in a spiral or meandering shape.
The plurality of permanent magnets 5 are installed such that N poles and S poles of adjacent permanent magnets arranged with a prescribed space on the yoke 6 are reversed each other. Thus, the voice coil 4 is positioned to face the permanent magnet 5. More specifically, it is configured that magnetic field generated between the N pole and the S pole of the permanent magnet passes properly through the vice coil 4.
In the flat speaker 11 described above, there is a problem in which the amplitude of the vibrating membrane 2 becomes so large that the vibrating membrane 2 hits the permanent magnet 5, when the input power in the regeneration band in the vicinity of resonance frequency becomes large, thus generating noises or deteriorating the vibrating membrane 2.
A corn-type speaker has a means for controlling large amplitude of vibration which is for example a corrugation (i.e., damper or spider) of cloth impregnated with phenol resin. However, it is difficult to provide with the same kind of corrugation in the flat speaker.
There are disclosed that a buffering sheet is arranged (in Japanese patent application publication 2001-333493) or a damper is installed (in the publication WO99/03304) in order to control the generation of noises by the hitting of the vibrating membrane and permanent magnet, or avoiding the deterioration of the vibrating membrane 2. FIG. 2 shows an example of providing a buffering sheet 12.
The buffer sheet 12 is attached an opposite face of the magnetic pole to the yoke 6 of the permanent magnetic 5. An air gap provided between the buffer sheet 12 and the vibrating membrane 2 enables to avoid the sound generated by the contact of the vibrating membrane with the permanent magnetic 5, and further more to secure the free vibration of the vibrating membrane 2.
The Japanese Utility application publication Hei 6-38400 discloses that an acoustic resistance sheet is adhered to a hole provided in a center portion of a case in a piezoelectric receiver to give appropriate braking of an acoustic resistance so that the frequency characteristic is planarized.
Since the vibrating membrane is held only by the edge portion in the flat speaker, the resonance of the vibrating membrane per se becomes large Q value (peak degree of the resonance) of the acoustic pressure characteristic becomes excessively large such as 3 to 5, thus the vibrating membrane hits the magnet.
Accordingly, in order to control the input in a low tone range in the conventional flat speaker, the input voltage is designed to be small, or the low tone range is cut off by the capacitor.
However, when the input voltage is designed to be small or the low tone range is cut off by the capacitor in order to control the input of the low tone range, there is a problem in which an output of the favorable acoustic quality may not be obtained.
Japanese Patent application publication 2003-284187 discloses a flat speaker in which a drive power is generated in the entire face of the planar vibrating membrane so that plane waves are produced as a plane sound source and the phase thereof is flat. In addition, since a paper is not used as in the conventional corn type speaker, the flat speaker is excellent in heat resistance and humidity resistance, thus enabling to operate inwater. Also since each soundcell is small, an acoustic feedback hardly occurs. The flat speaker has a remarkable feature in which the thickness is very thin such as up to 1 cm, the weight is light, and the construction is very simple with small number of components.
FIG. 18 shows a cross sectional view of an example of another conventional flat speaker. The flat speaker 500 as shown in FIG. 18 includes a flat plate type yoke 501 formed by an iron plate (i.e., ferromagnetic metal plate) and a plurality of permanent magnet 503 fixed on one face of the yoke 501 with respective magnetic axes vertically placed. The permanent magnets 503 are arranged with a specific interval in a direction along the planer face of the yoke in such manner that polar characters of the adjacent magnets are reversed each other.
Furthermore, the flat speaker 500 includes an edge portion 505 having an arched portion 506 and jointed with a shelf portion 501b on a surrounding wall portion 501a of the yoke 501, and a vibrating plate 507 jointed by an adhesive material such as adhesive agent through the edge portion 505 and movably supported with a specific distance apart from the surfaces of the magnetic poles of the permanent magnet 503. FIG. 18 shows an edge portion 505 which is depicted with slanted lines.
The vibrating plate 507 is formed by an insulating base film or the like, and voice coils with at least one spiral forms in correspondence to the respective permanent magnets are formed on one or both faces of the vibrating plate 507. All the voice coils 509 are connected in such manner that currents flow in the same direction in the adjacent sides of the adjacent voice coils 509. A plurality of holes (not shown) are formed between the permanent magnets 503 in the yoke 501 in order to adjust an air pressure generated together with vibration of the vibrating plate 507. The lead portion 513 is extracted from the voice coil 509, and connected to the electric supply line 511 provided at the end portion of the lead portion 513.
FIG. 19 shows a plan view of the flat speaker 500. The edge portion 505 in a frame shape is jointed with the yoke 501 corresponding to the shape thereof, and the vibrating plate 507 is joined with the edge portion 505. Total of five voice coils in the longitudinal direction and three voice coils in the perpendicular direction are densely arranged in the same plane on the vibrating plate 507. A voltage is applied through an electric supply line to the voice coil.
Current flows through respective voice coils 509 of the flat speaker 500 to drive the vibrating plate 507 vertically upward and downward across thereof. The lead portion 513 is extracted from the voice coil 509, and connected to the electric supply line 515 provided at the end portion of the lead portion 513.